HTML代码块：

<textarea class="weui_textarea" id="textarea" maxlength="200"></textarea>
<div class="data-count"><span id="count">0</span>/200</div>

jQuery方法：

$('#textarea').on('input', function(){
    var text = $(this).val();
    var len = text.length;
    $('#count').text(len);
});

JS方法:

var el = document.getElementById('textarea')
el.addEventListener('input',function () {
    var val = el.value;
    var len = val.length; 
    document.getElementById('count').innerHTML = len